An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Usually, the liquid crystal display device needs a power supply circuit to provide a working voltage.
Referring to FIG. 6, a typical art power supply circuit 10 for an LCD (not shown) includes a control signal input terminal 11 which is configured for receiving control signals, an output terminal 12 for providing an operation voltage for the LCD, a five volt direct current (DC) power supply 13, a first transistor 14, a second transistor 15, a filter capacitor 16, and a resistor 17.
The first transistor 14 is a p-channel metal-oxide-semiconductor field-effect transistor (MOSFET). A gate electrode 141 of the first transistor 14 is connected to the five volt DC power supply 13 via the resistor 17. A source electrode 142 of the first transistor 14 is connected to the five volt DC power supply 13. A drain electrode 143 of the first transistor 14 is connected to the output terminal 12.
The second transistor 15 is a negative-positive-negative (NPN) bipolar transistor. A base electrode 151 of the second transistor 15 is connected to the control signal input terminal 11. An emitting electrode 152 of the second transistor 15 is connected to the gate electrode 141 of the first transistor 14. A collecting electrode 153 of the second transistor 15 is grounded.
A working principle of the power supply circuit 10 for the LCD is described as follows. When the LCD is connected up a commercial power, the five volt DC power supply 13 provides a five volt voltage to the source electrode 142 of the first transistor 14. When the LCD is powered on, an electric potential of the control signal input terminal 11 is a logic high electric potential. The second transistor 15 is switched on, and the gate electrode 141 of the first transistor 14 is grounded via the collecting electrode 153 and the emitting electrode 152 in turn. Therefore, the first transistor 14 is switched on, a five volt voltage of the five volt DC power supply 13 is provided to the output terminal 12 via the source electrode 142 and the drain electrode 143.
When the LCD is powered off, an electric potential of the control signal input terminal 11 is a logic low electric potential. The second transistor 15 is switched off. The five volt DC power supply 13 provides a voltage to the gate electrode 141 of the first transistor 14 via the resistor 17. Therefore, the first transistor 14 is switched off, and the five volt DC power supply 13 stops providing voltage for the output terminal 12.
When the first transistor 14 is switched on, and the five volt voltage is provided to the output terminal 12 via the activated first transistor 14, a rush current is generated at the moment that the first transistor 14 is switched on. The rush current may accelerate an aging process of electronic devices of the LCD. Thus a service life of the LCD is liable to be reduced.
Further, in case that the LCD is short-circuited, a high short circuit current passes through the first transistor 14. Thus, the first transistor 14 is liable to be destroyed. Thus the reliability of the power supply circuit 10 is low.
It is desired to provide a new power supply circuit used in an LCD which can overcome the above-described deficiencies.